The objective of the present work is to investigate correlations between ionic fluxes taking place across the sodium pump on the one hand, and metabolic conditions prevailing inside the cell, on the other hand. A quantitative description of these correlations will help throw light on the molecular mechanisms underlying both the observed ion fluxes and the accompanying biochemical events. The sodium pump is known to shift from the normal Na:K exchanging mode of operation to a Na:Na exchanging one whenever intracellular (ADP) is raised (or when the ATP/ADP concentration ratio is lowered). The kinetics of this interesting behavior will be investigated, mainly on the squid giant axon, using the internal dialysis technique, with (1) experimental control of electrical, metabolic and ionic parameters and (2) measurement of the five variables: Na and K in- and effluxes, and ATP hydrolysis. Using equilibrium constants for magnesium-nucleotide complexes and various nucleotide (myokinase, phosphagen kinase) equilibria obtained in vitro on artificial axoplasm internal perfusion media will be prepared whose composition with respect to ATP, ADP, AMP and their metal complexes is exactly known. First, the relationship between internal nucleotide composition and mode of operation (Na:K vs. Na:Na) of the pump will be clarified. Then, when the conditions for predominantly Na:Na and predominantly Na:K mode will have been established, a detailed kinetic analysis will be made of either mode of operation with respect to the pertinent substrates and products (Nai, Ki, Nao, Ko).